Background: This group studied the three-dimensional structure and function of biological macromolecules (proteins, enzymes, and nucleic acids) using primarily the method of single-crystal x- ray diffraction. Objective: Major emphasis was given to macromolecular complexes and complex interactions between macromolecules. Systems studied include a model system for DNA replication and a large human protease complex. Structural studies of retroviral proteins and enzymes were undertaken to understand the basic biology of retroviruses and to exploit their use as potential drug targets. Human skin proteins and auto-antigens were studied to understand better the molecular basis for skin disorders and the etiology of autoimmunity disorders such as Lupus. Results during the past year: DNA replication: We reported the high- resolution crystal structures of the helicase assembly/gene 59 protein from phage T4 and proposed a model for the binding of forked DNA substrates and putative interaction sites within the DNA replisome complex. This novel protein structure may serve as an archetype for understanding the function of other DNA-binding proteins. Retroviral proteins: We reported the crystal structures of an active two domain form of Rous sarcoma virus integrase. Based on novel interactions observed between the C-domains, we have constructed a new model for protein:protein and protein:DNA interactions at the center of the retroviral pre-integration complex. Understanding these interactions may provide important leads for develop new therapies for diseases such as AIDS. Autoantigens: we have engineered, expressed and purified several new forms of the human La protein and completed crystallization screening tests. Skin proteins: We have determined the high-resolution structure of human transglutaminase III and are completing the analysis of its structural features. Conclusions and Significance: A long-term goal of our research is to understand better the process of DNA replication by studying the structures of the molecular machinery involved. We believe that our recently determined structure of GP59 will be important for understanding how other proteins and enzymes assemble at a replication fork. The structure of the two-domain form suggests models by which the protein interacts with both viral and host cell DNA and provides a new view of the critically important active center. Detailed structural information from transglutaminase III will help us understand the basis of action and substrate specificity and their roles in human skin disorders.